The present invention relates to semiconductor manufacturing. More specifically, the present invention relates to vertical wafer boats having Y shaped column racks (or columns) made from a single casting.
Although other materials may be used, e.g., Silicon-Germanium (SiGe) or Galium Arsenide (GaAs), Silicon (Si) is presently the most important semiconduct or for the electronics industry. Very Large Scale Integrated (VLSI) circuit technology (i.e., up to about 100,000 devices per chip), and Ultra Large Scale Integrated (ULSI) circuit technology (i.e., more than 100,000 and in some cases exceeding one billion devices per chip) being based almost entirely on silicon.
It is critical that the fabrication of VLSI and ULSI circuits which take place on silicon substrates possess very high crystalline perfection or purity. That is, in crystalline solids, the atoms which make up the solid are spatially arranged in a periodic fashion. If the periodic arrangement exists throughout the entire solid, the substance is defined as being formed of a single crystal. The periodic arrangement of the atoms in the crystal is called the lattice. Very high crystalline perfection requires that the silicon substrate possess a minimum of impurities and structural defects throughout its single crystal silicon lattice.
Generally, raw material, e.g., quartzite, is refined into electronic grade polysilicon (EGS) and melted. A silicon seed crystal is than used to grow a single crystal silicon ingot from the molten EGS. The ingot is than precisely sliced and polished into silicon wafers. The silicon wafers provide the substrates upon which VLSI and ULSI circuits are ultimately built through a complex sequence of wafer fabrication processes.
The increasing size of silicon wafers is one of the most obvious trends in silicon material technology. Presently, 300 mm diameter wafers are expected to ultimately replace most 150 mm and 200 mm wafer applications. It is also predicted that 400 mm wafers will probably be introduced in the not too distant future. The use of larger diameter wafers for maintaining productivity presents several major challenges to semiconductor manufactures. For example, facilities with equipment capable of handling the larger wafers, e.g., vertical furnaces, must be built. New patterning techniques must be developed to print smaller feature sizes over larger areas. The larger wafers must also be thicker to increase their resistance to warping and other structural deformations. Moreover, the larger wafers are also heavier, requiring the use of automated wafer transport systems.
As the silicon wafers become bigger and heavier, the problem of preventing impurities and structural defects to the lattice, i.e., of maintaining very high crystalline perfection, becomes even more critical. Two such structural defects, which becomes especially problematic in 300 mm silicon wafers and larger, are that of xe2x80x9cback side damagexe2x80x9d and xe2x80x9cslipxe2x80x9d in the lattice structure.
Back side damage is when a wafer moves across a surface of a wafer support device, causing scratches in the back side of the wafer.
Slip in silicon wafers is a function of the stress applied to the wafer. This stress can be mechanical (e.g., frictionally induced) and/or thermal. As the wafers are stressed, the crystal lattice undergoes elastic deformation that disappears as the solid crystal returns to its original position upon release of the stress. However, severe stress leads to slip, which is the plastic or permanent deformation in the crystal lattice, which remains when the stress is released. Slip occurs when the elastic limit (or yield strength) of the silicon is exceeded and the lattice becomes permanently misaligned.
Slip is common during high temperature processing of silicon wafers in heat treatment furnaces (furnacing operations), as thermal stress is proportional to the processing temperature. The transition temperature from brittle to ductile behavior of the wafer is generally within the range of about 720 to 1000 degrees C. Therefore slip, whether induced by thermal or mechanical stress, becomes especially problematic at process temperatures above 720 degrees C.
Wafer boats are wafer support devices, which are subjected to furnacing operations during semiconductor wafer processing. Horizontal wafer boats are typically designed to support a horizontal row of wafers, which are inserted into a horizontal furnace tube for high temperature processing. Vertical wafer boats are typically designed to support a vertical stack of wafers, which are inserted into a vertical furnace tube. Generally, for large diameter silicon wafers, e.g., 300 mm, vertical wafer boats are more commonly used. This is because vertical furnaces have a smaller foot print than horizontal furnaces and therefore take up less of the expensive manufacturing space. Additionally, vertical furnaces generally demonstrate better temperature control than horizontal furnaces.
Wafer boats are generally composed of ceramic materials. Ceramic materials, which are joined by ionic or covalent bonds, are typically composed of complex compounds containing both metallic and non metallic elements. Ceramics typically are hard, brittle, high melting point materials with low electrical and thermal conductivity, good chemical and thermal stability, and high compressive strengths. Examples of ceramic materials are quartz, silicon carbide (SiC) and recrystalized SiC. One such recrystalized SiC is available from Saint-Gobain Ceramics and Plastics, Inc. of Worcester, Mass., under the trade name CRYSTAR(copyright). This material is a silicon carbide ceramic that has been impregnated with high purity silicon metal.
Referring to FIG. 1, a typical prior art vertical wafer boat 10 generally includes three or four support rods 12 extending vertically upwards from a generally circular horizontal base 14, and spaced radially along the periphery of the base. The rods 12 have a plurality of cantilevered wafer support arms (or teeth) 16 supported only at one end, which extend inwardly toward the center of the base 14 to define a series of slots therebetween. The slots are sized to receive the silicon wafers, which are supported by the arms 16 during furnacing operations.
Problematically for larger wafers, the prior art wafer support arms 16 provide most of their support at the outer periphery of the wafer. Accordingly, most of the weight of the wafer is unsupported and distributed toward its center. Therefore, during high temperature thermal processing, the center of the wafers tend to sag, promoting slip in the crystal lattice of the wafer.
Because of the geometry of the circular wafers, substantially half of the weight of the wafer, i.e., the inner wafer weight, is distributed within a circular area that is 70 percent the radius (R) of the wafer. Conversely, half of the weight of the wafer, i.e., the outer wafer weight, is distributed over a doughnut shaped area that has an inner radius of 0.7 R and an outer radius of 1.0 R. As a result, supporting the wafers at or about the 0.7 R circular boundary region of a wafer, e.g., from 0.6 R to 0.8 R, balances the inner and outer wafer weights and greatly reduces the potential for sagging during high temperature thermal processing.
Current prior art boat designs require deep slots, thereby making the arms 16 of the support rods 12 long enough to extend to the 0.7 R point. However, manufacturing this geometry is cumbersome due to the precise machining required and the inherently low yield rates. Also the added length of the cantilevered arms impose a large moment force at the single support point where the arm attaches to the rod body, unduly increasing the probability of failure or breakage. Moreover, because the arms provide support at only three or four small discrete areas on the wafers, the possibility of back side damage is enhanced for the heavier wafers.
Ideally, providing actual structural support at the center region of a wafer would eliminate the problem of sag in the center. However, the above problems associated with support arms 16 extending to the 0.7 R point of a wafer, become even more pronounced if the arms 16 are required to reach further to the center point of the wafer. Additionally, the longer arms would make it very difficult for conventional transfer equipment to reach and move the wafers in an out of the slots.
One prior art attempt to solve this problem, is to provide a plurality of discrete circular ceramic rings. The rings may be sized to support the 0.7 R region of the wafers, e.g., 210 mm rings for 300 mm wafers, or the edge region of the wafers, e.g., edge rings. The rings would be slid into each slot and the wafer""s would then rest upon each ceramic ring. However, the rings greatly add to the cost of the boat. Additionally, the rings essentially enclose the support area where the wafers rest, making it difficult for conventional transfer equipment to remove the wafers from the slots. Also, since the rings typically add up to 100 additional discrete moving parts to the boats (one for each slot), the potential for generating damaging microscopic particles, or causing back side damage, is greatly enhanced.
One such apparatus designed to support the wafers at the 0.7 R boundary without using discrete rings is disclosed in the above cross-referenced U.S. Patent Application tilted xe2x80x9cA Wafer Boat With Arcuate Wafer Support Armsxe2x80x9d. In that design, a pair of opposing arcuate column racks are utilized to support the wafers at the 0.7 R region.
Another problem associated with current prior art wafer boats is that the complexity of their design requires them to be manufactured from discrete separate components, e.g., top plates, bases, rods, columns (or column racks) and tabs. The separate components are generally cast individually and then assembled by welding or other fastening means. Because of the varying cross sections of the existing designs, casting or extruding is not possible for an entire assembly, thereby greatly adding to the cost of the prior art wafer boats.
Accordingly, there is a need for an improved wafer boat, which can provide a single cast construction with enhanced support to reduce wafer sag in large diameter silicon wafers, while also providing maximum openness for material handling at a reasonable cost.
The present invention offers advantages and alternatives over the prior art by providing a wafer boat having a column rack for stacking silicon wafer. The column rack having a generally Y shaped cross section.
The invention provides a substantial number of advantages over the prior art. The Y cross sectional shape provides enhanced support for both the 0.7 R regions and the central regions of a wafer. Additionally, the column rack and/or the entire wafer boat may be manufactured from a single casting or a single extrusion.
These and other advantages are accomplished in an exemplary embodiment of the invention by providing a vertical ceramic wafer boat for supporting a silicon wafer having a predetermined radius R. The wafer boat comprises a base portion and a column rack, which extends generally vertically upwards from the base portion. The column rack includes a pair of vertical column rack supports extending generally vertically upwards from the base portion. The column rack also includes a plurality of wafer supports having a generally Y shaped cross section. The wafer supports extend substantially horizontally from the column rack supports to define a plurality of slots within the column rack sized to receive the wafer.
In an alternative embodiment of the invention, the wafer supports of the wafer boat comprise first and second wafer support legs each forming a branch of the Y shaped cross section. The first and second legs have substantially equal lengths greater than the radius R of the wafers. Each leg extends inwardly from the vertical column rack supports from an anchored distal end. A third wafer support leg forms a third branch of the Y shaped cross section and terminates in a free distal end.
In another embodiment the column rack is formed from a single casting. Additionally, the base portion may be have a generally Y shaped cross section and the entire wafer boat may be formed from a single casting.
In another alternative embodiment of the invention, the wafer supports of the wafer boat include a plurality of raised pads spaced to provide support for the 0.7 R region of the wafer. Additionally, the wafer supports may also include a raised center pad located to provide support for the central region of the wafer.